The present invention relates to a wheel support hub unit adapted to support rotatably a wheel of, for example, an automobile on a suspension unit, a bearing ring member for a wheel support hub unit, and a method of manufacturing the same.
A wheel of an automobile and the like is supported rotatably on a wheel support hub unit fixed to a suspension unit for a vehicle body. The wheel support hub unit is provided with a plurality of rolling elements interposed between an inner member and an outer member, which have raceway surfaces, the inner member and outer member being provided with fixing flanges to be fixed to the suspension unit for the wheel or vehicle body. A positioning unit adapted to position the wheel is provided on a side of the fixing flanges.
When the inner member and outer member as bearing ring members are manufactured, the surface area of the fixing flanges is large, and a very large forming load is needed in the cold forging process, so that the hot forging process is generally carried out. However, in the hot forging process, the dimensional accuracy is low, and a decarbonization removal operation is necessary in some cases in a thermal treatment unit. Since a large cutting margin is necessary in a later process, the manufacturing cost increases.
Under the circumstances, a related art bearing ring member manufacturing method in which fixing flanges are cut and raised from an intermediate raw material formed by cold forging a cylindrical pipe as a base material with the portions left after the cutting and raising operations used as positioning portions (refer to, for example, the Patent Document 1), and a related art bearing ring member manufacturing method in which positioning portions are formed separately from the bearing ring member and fixed to the bearing ring member (refer to, for example, to the Patent Document 2) are proposed.
A hub ring 6′ shown in FIG. 14 and an outer ring 3 shown in FIG. 15 are known as a bearing ring member constituting the inner member and outer member in which consideration is given to the reduction of the weight thereof and the retention of the strength thereof. The hub ring 6′ is provided with a solid shaft portion 5′ and radial wheel fixing flanges 7′ extending in the direction crossing the shaft portion 5′ at right angles thereof. In the wheel fixing flanges 7′, a plurality of fixing portions 21′ are formed to a large thickness, and, among the fixing portions 21′, web-shaped small-thickness portions 22′ are formed. The outer ring 3 is provided with a hollow shaft portion 15, and a suspension unit fixing flanges 18 extending in the direction crossing the shaft portion 15 at right angles thereto, the suspension unit fixing flanges 18 being made heteromorphous at an outer circumferential portion thereof.
These bearing ring members are made by related art methods as shown in FIG. 16, by forming small-thickness portions 22′ by expanding the excess parts among the fixing sections 21′ of the wheel fixing flanges 7′ by a hot forging operation, and then cutting off unnecessary parts 23 by subjecting the small-thickness portions 22′ to a trimming process as shown in FIG. 17.    [Patent Document 1] JP-A-2003-25803    [Patent Document 2] JP-A-2003-291604    [Patent Document 3] JP-A-2004-74815
However, in the invention disclosed in the above Patent Publication 1, a bending process is carried out when a joint portion between the fixing flanges, which need the highest strength, and the outer circumferential portion of the bearing ring member is cut and raised, flaws, such as cracks being liable to occur, the strength of the joint portion becoming unstable. Moreover, since the joint portion has to be rendered easily bendable, it is impossible to reinforce this portion by, for example, increase the thickness thereof and the like. Since the positioning portion is not formed to a circumferentially continuous cylindrical shape, the positioning operation becomes unstable. The strength of the joint portion is not sufficiently high, either.
In the patent disclosed in the above Patent Document 2 and that disclosed in the Patent Document 3, the positioning portion is formed separately from the bearing ring member, so that the positional accuracy and a degree of right angle of the positioning portion and raceway surface on the side of the bearing ring member is liable to lower. Moreover, the number of parts increases to cause the manufacturing cost to be heightened. Furthermore, since a bearing ring member including fixing flanges cannot be cold formed, a cutting margin in a later step is large, so that the manufacturing cost increases.
The bearing ring members shown in FIGS. 14A and 14B and FIGS. 15A and 15B are manufactured by the hot forging, the above-mentioned problems are included. Such parts of the bearing ring members that are pressed to a small thickness extend greatly toward the outer circumferential side thereof, so that the quantity of excess thickness becomes large to cause the yield to lower. Moreover, since the excess thickness is large, the surface area of the upset portion increase, and the forming load increases. For these reasons, it becomes disadvantageous in particular to form such a heteromorphous flanges by the cold forging in which the stress becomes high.